Command Design Pattern

Intent

Encapsulate a request as an object, thereby letting you parametrize
clients with different requests, queue or log requests, and support
undoable operations.

Promote "invocation of a method on an object" to full object status

An object-oriented callback

Problem

Need to issue requests to objects without knowing anything about the
operation being requested or the receiver of the request.

Discussion

Command decouples the object that invokes the operation from the one
that knows how to perform it. To achieve this separation, the designer
creates an abstract base class that maps a receiver (an object) with an
action (a pointer to a member function). The base class contains an
execute() method that simply calls the action on the receiver.

All clients of Command objects treat each object as a "black box" by
simply invoking the object's virtual execute() method whenever the
client requires the object's "service".

A Command class holds some subset of the following: an object, a
method to be applied to the object, and the arguments to be passed when
the method is applied. The Command's "execute" method then causes
the pieces to come together.

Sequences of Command objects can be assembled into composite (or macro)
commands.

Structure

The client that creates a command is not the same client that executes
it. This separation provides flexibility in the timing and sequencing
of commands. Materializing commands as objects means they can be
passed, staged, shared, loaded in a table, and otherwise instrumented
or manipulated like any other object.

Command objects can be thought of as "tokens" that are created by
one client that knows what need to be done, and passed to another
client that has the resources for doing it.

Example

The Command pattern allows requests to be encapsulated as objects,
thereby allowing clients to be parametrized with different requests.
The "check" at a diner is an example of a Command pattern. The waiter
or waitress takes an order or command from a customer and encapsulates
that order by writing it on the check. The order is then queued for a
short order cook. Note that the pad of "checks" used by each waiter is
not dependent on the menu, and therefore they can support commands to
cook many different items.

Check list

Define a Command interface with a method signature like execute().

Create one or more derived classes that encapsulate some subset
of the following: a "receiver" object, the method to invoke, the arguments to pass.

Instantiate a Command object for each deferred execution request.

Pass the Command object from the creator (aka sender) to the invoker (aka receiver).

The invoker decides when to execute().

Rules of thumb

Chain of Responsibility, Command, Mediator, and Observer, address how
you can decouple senders and receivers, but with different trade-offs.
Command normally specifies a sender-receiver connection with a
subclass.

Chain of Responsibility can use Command to represent requests as
objects.

Command and Memento act as magic tokens to be passed around and invoked
at a later time. In Command, the token represents a request; in
Memento, it represents the internal state of an object at a particular
time. Polymorphism is important to Command, but not to Memento because
its interface is so narrow that a memento can only be passed as a
value.

Command can use Memento to maintain the state required for an undo
operation.

MacroCommands can be implemented with Composite.

A Command that must be copied before being placed on a history list acts
as a Prototype.

Two important aspects of the Command pattern: interface separation
(the invoker is isolated from the receiver), time separation (stores
a ready-to-go processing request that's to be stated later).